Shower tray waste type & stench from toilet flush

[Greenjefe]

Hi all, I have a corner en suite shower tray that has a 5" long x 1" wide narrow slot as the drain, letter box shape. Has anyone any idea what makes this tray might be. There is a problem with the waste trap, it's as if there isn't one fitted, it must be linked to the toilet outfall, there is a stench from the shower drain every time the toilet flushes and you can hear the water gushing through the shower drain. 
I get the feeling I may need to remove the shower tray to investigate what type waste trap may be fitted in case it is damaged or non existent. This is a Redrow build BTW so has anyone had any experience with this type shower basin waste fitting or what the manufacturer is should I need to replace it or if the waste connection is a bespoke type. 

[Iceverge]

Broken AAV. Air admittance valve. 

[ProDave]

Most shower wastes built in like that are "top access" and they have some form of "bucket" that lifts out from the top, it is this lift out but that forms the trap and you can lift it out to clean it.

 

If all you see looking down the rectangular hole is the pipe heading off, it might have a part missing and hence not forming a trap so the smells get in.

[stephanh]

had a similar problem with a bath waste - the waste was a rectangular box shape which allowed a syphoning when high winds crossed over the SVP

Sounds as though the 'plug' of water from flushing is syphoning your waste - need to replace the trap for a better one.